Some power-split hybrid vehicles split power from a primary power source, such as an engine, by operatively connecting the power source to a member of differential gearing, such as a planetary gear set. In such an arrangement, different modes of operation are available, including a hybrid operating mode in which the engine is able to operate at or near its most efficient operating speed and torque, while one or more electric motor/generators connected to another member or other members of the differential gearing are controlled to meet vehicle speed and torque demands.
In some such hybrid vehicles, when a motor/generator must operate as a motor to control a member of the differential gearing, it requires electrical power from another power source. Hybrid vehicles with two motor/generators may utilize the other motor/generator to operate as a generator, providing electrical power to power the motor/generator that must operate as a motor. This may create a “power loop” in which energy must be converted from mechanical energy, to electrical energy, and then back to mechanical energy at some point upstream of the general flow of power through the system, with related power losses inherent with the increased conversion and transmission of energy, or a similar situation where the generation of electrical power adds to the load upon the motor/generator that must operate as a motor. In some hybrid configurations, the power loop may be particularly inefficient as it may retard the desired motion of a wheel axle.